1. Field of the Invention
This invention relates to photothermographic materials and in particular to speed enhancing and post-processing stabilization of photothermographic silver-containing materials.
2. Background of the Art
Silver halide containing photothermographic imaging materials processed with heat, and without liquid development have been known in the art for many years. These materials, also known as dry silver materials, generally comprise a support having thereon a photographic light-sensitive silver halide, a light-insensitive organic silver salt, and a reducing agent for the organic silver salt.
The light-sensitive silver halide is in catalytic proximity to the light-insensitive organic silver salt so that the latent image, formed by irradiation of the silver halide, serves as a catalyst nucleus for the oxidation-reduction reaction of the organic silver salt with the reducing agent when the emulsion is heated above about 80.degree. C. Such media are described, for example, in U.S. Pat. Nos. 3,457,075; 3,839,049; and 4,260,677. The silver halide may also be generated in the media by a preheating step in which halide ion is released to form silver halide.
A variety of ingredients may be added to these basic components to enhance performance. For example, toning agents may be incorporated to improve the color of the silver image of the photothermographic emulsions, as described in U.S. Pat. Nos. 3,846,136; 3,994,732 and 4,021,249. Various methods to produce dye images and multicolor images with photographic color couplers and leuco dyes are known and described in U.S. Pat. Nos. 4,022,617; 3,531,286; 3,180,731; 3,761,270; 4,460,681; 4,883,747 and Research Disclosure, March 1989, item 29963.
A common problem that exists with photothermographic systems is post-processing instability of the image. The photoactive silver halide still present in the developed image may continue to catalyze print-out of metallic silver during room light handling or exposure to heat or humidity. Thus, there exists a need for stabilization of the unreacted silver halide. The addition of separate post-processing image stabilizers has been used to impart post-processing stability. Most often these are sulfur containing compounds such as mercaptans, thiones, and thioethers as described in Research Disclosure, June 1978, item 17029. U.S. Pat. Nos. 4,245,033; 4,837,141; and 4,451,561 describe sulfur compounds that are development restrainers for photothermographic systems. Mesoionic 1,2,4-triazolium-3-thiolates as fixing agents and silver halide stabilizers are described in U.S. Pat. No. 4,378,424. Substituted 5-mercapto-1,2,4-triazoles such as 3-amino-5-benzothio-1,2,4-triazole as post-processing stabilizers are described in U.S. Pat. Nos. 4,128,557; 4,137,079; 4,138,265; and Research Disclosure, May 1978, items 16977 and 16979. U.S. Pat. No. 5,158,866 describes the use of omega-substituted 2-propionamidoacetyl or 3-propionamidopropionyl stabilizer precursors as post-processing stabilizers in photothermographic emulsions. U.S. Pat. No. 5,175,081 describes the use of certain azlactones as stabilizers.
Problems arising from the addition of stabilizers may include thermal fogging during processing and losses in photographic speed, maximum density or contrast at effective stabilizer concentrations.
Stabilizer precursors are materials which have blocking or modifying groups that are usually cleaved during processing with heat and/or alkali. The cleaving provides the primary active stabilizer which can combine with the photoactive silver halide in the unexposed and undeveloped areas of the photographic material. For example, in the presence of a stabilizer precursor in which a sulfur atom is unblocked upon processing, the resulting silver mercaptide will be more stable than the silver halide to light, atmospheric, and ambient conditions.
Various blocking techniques have been utilized in developing the stabilizer precursors. U.S. Pat. No. 3,615,617 describes acyl blocked photographically useful stabilizers. U.S. Pat. Nos. 3,674,478 and 3,993,661 describe hydroxyarylmethyl blocking groups. Benzylthio releasing groups are described in U.S. Pat. No. 3,698,898. Thiocarbonate blocking groups are described in U.S. Pat. No. 3,791,830, and thioether blocking groups in U.S. Pat. Nos. 4,335,200; 4,416,977; and 4,420,554. Photographically useful stabilizers which are blocked as urea or thiourea derivatives are described in U.S. Pat. No. 4,310,612. Blocked imidomethyl derivatives are described in U.S. Pat. No. 4,350,752, and imide or thioimide derivatives are described in U.S. Pat. No. 4,888,268. Removal of all of these aforementioned blocking groups from the photographically useful stabilizers is accomplished by an increase of pH during alkaline processing conditions of the exposed imaging material.
Thermally sensitive blocking groups are also known. These blocking groups are removed by heating the imaging material during processing. Photographically useful stabilizers blocked as thermally sensitive carbamate derivatives are described in U.S. Pat. Nos. 3,844,797 and 4,144,072. These carbamate derivatives presumably regenerate the photographic stabilizer through loss of an isocyanate. Hydroxymethyl blocked photographic reagents which are unblocked through loss of formaldehyde during heating are described in U.S. Pat. No. 4,510,236. Development inhibitor releasing couplers releasing tetrazolyolthio moieties are described in U.S. Pat. No. 3,700,457. Substituted benzylthio releasing groups are described in U.S. Pat. No. 4,678,735. U.S. Pat. Nos. 4,351,896 and 4,404,390 utilize carboxybenzylthio blocking groups for mesoionic 1,2,4-triazolium-3-thiolate stabilizers. Photographic stabilizers that are blocked by a Michael-type addition to the carbon-carbon double bond of either acrylonitrile or alkyl acrylates are described in U.S. Pat. Nos. 4,009,029 and 4,511,644, respectively. Heating of these blocked derivatives causes unblocking by a retro-Michael reaction.
Various disadvantages attend these different blocking techniques. Highly basic solutions necessary to cause deblocking of the alkali sensitive blocked derivatives are corrosive and irritating to the skin. With photographic stabilizers that are blocked with a heat removable group, it is often found that the liberated reagent or by-product can react with other components of the imaging construction and cause adverse effects. Also, inadequate or premature release of the stabilizing moiety during heat processing may occur.
There has been a continued need for improved post-processing stabilizers or stabilizer precursors that do not fog or desensitize photothermographic materials, and for stabilizer precursors that release the stabilizing moiety at the appropriate time and do not have any detrimental effects on the photosensitive material or user of said material.
Photolytically active stabilizer precursors for photothermographic silver imaging compositions which apparently release bromine atoms are described in U.S. Pat. No. 4,459,350.
U.S. Pat. No. 4,207,108 describes the use of thione compounds as a photographic speed enhancing additive, U.S. Pat. No. 4,873,184 describes the use of metal chelating agents to enhance speed in silver halide systems, and U.S. Pat. No. 4,264,725 describes the use of benzyl alcohol and 2-phenoxyethanol as speed enhancing solvents for photothermographic materials.
Stabilizer precursors of this type can be added to photothermographic formulations without the necessity of rebalancing the formulation to compensate for effects on sensitometry, as is often the case with other stabilizers in the art.